Med Calc: Measurement Systems

Unit 2 Measurement Systems and Conversion Processes Metric or Systeme Internationale: A system of measurement based on multiples of 10. The basic unit of volume is the liter, the basic unit of weight is the gram, and the basic unit of length is the meter. Apothecary System: A system of measurement in which the basic unit of weight is the grain from the Latin granum, grain or seed; based on the weight of one grain of wheat. The basic unit of fluid volume in this system is the minim from the Latin minima, meaning smallest or least. Household: This is a system of measure much more familiar to us. Each day we use common measures in cooking or measurement. Note that Americans have not adopted more international measures and speak in “household” terms. When counseling clients about medication dosing, it is important to translate medical terminology into volumes readily understood by the patient. Most Moms don’t know how much 5 cc is but can easily identify a teaspoon. Other units of measure include units (common for insulin and heparin), milliequivalents (common for some drugs such as potassium chloride), and international units (common for vitamins). In nursing and health care, the physical quantities associated with pharmacology can be expressed in terms of four basic dimensions. These are weight, volume, length, and temperature. For example, doses may be measured in units of milligrams, grains, or micrograms, but regardless of the units used, doses are always weights. Similarly, the units of how doses are administered to our patients are volumes such as teaspoons, milliliters, or fluid drams. Applying pharmacology math requires the nurse to know three primary systems of measurement (metric, apothecary, and household) and how to convert within and between these systems of measurement. Go back and examine your conversion chart. The conversions between the measures of these 3 systems are approximately the same…not exact. Note that 5 mL is = to 5 cc but will be ~ (approximately) 60 minums and ~ (approximately) 1 teaspoon. Within a column, the values are equal but between columns, values are approximate.

Metric System The metric system is commonly used for meassssurement of volume, weight and length in the health care professions. Note that on the conversion chart, the first 7 rows of conversions measure volumes; the next 5 rows are measures of weight; and the last is a commonly used measure of length.

The metric system uses a collection of prefixes in combination with the basic units to indicate changes in the values. The same prefixes are used for units of volume, weight and length but the basic units vary. Basic Unit of Volume = Liter (abbreviated “L” as a capital letter, not lower case) Basic Unit of Weight = gram (g, gm, GM or Gm) (NOTE: There is NO “r” in ANY abbreviation for gram!) Basic Unit of Length = meter ( m) Commonly used prefixes are: Larger than the basic unit = kilo (k) Smaller than the basic unit = centi (c) (eg: Centimeter = cm) = milli (m) (eg: Milliliter = mL) (always use upper case L) = micro (mc) (eg: Microgram = mcg) What is a cc? To answer that question, let’s start by looking at a cube.

As you can see each side is one centimeter long so that the total volume of the cube is 1. One what? It isn’t 1 centimeter – that’s the length of just one side. It isn’t “1 square centimeter” either, that’s the area of one face. The volume of the cube is “1 cubic centimeter” (1 cc). The amount of liquid that would fill the cube is “1 millimeter” (1 ml). Therefore 1 cc = 1 ml.

APOTHECARY SYSTEM The apothecary system is one of the oldest used systems of measurement. Drug manufacturers and health care providers use this system to order drugs that have been in use for many years. These include: Morphine Codeine Atropine Aspirin Phenobarbital Roman numerals are often used in this system. Remember that when smaller Roman numerals follow a larger one the number is added to make numbers larger. For example, x (is 10) and xii (is 10 + 2 or 12). Remember that when smaller Roman numerals precede larger numerals, you must subtract the numbers from the larger. For example, x (is 10) and ix (is 10 – 1 or 9). Review Roman numerals in your text if you need more review. You should know Roman numerals i, ii, iii, iv, v, vi, vii, viii, ix, x, and xv.

The symbol ss is used for the fraction ½. In the Apothecary system, volume is measured using the minim and the fluid dram. 15 minims ~ 15 gtt ~ 1 mL ( the symbol ~ reminds you that this is approximately the same as the other values, not = however, we do treat them as approximately equal) 60 minims ~ 1 dram 8 fluid drams ~ 1 fluid ounce Units of Weight: The basic unit of weight is the grain (gr) gr 15 ~ 1 g 60mg ~ 1 gr 1 dram ~ 60 gr Conversions between systems: 1 1 1 1

dram ~ 5ml ounce ~ 30 ml pint ~ 500 ml quart ~ 1000ml

APOTHECARY/METRIC CONVERSION CLOCK The Curren and Munday textbook (pg 46) illustrates a quick and convenient means of converting mg and gr values. Using the face of a clock you can visualize the placement of 60 mg at 12 o’clock. The 60 mg (60 minutes) is approximately gr i (1 hr). Note that 15 mg appears at 15 minutes and gr ¼ on the grain scale. At 30 minutes you will find 30 mg and gr ½; at 45 minutes, you will find 45 mg and gr ¾; at 10 minutes you will find 10 mg and gr 1/6. This is a very convenient way to see the relationship between mg and gr (gr i = 60 mg).

HOUSEHOLD SYSTEM The household system is the least accurate of the three systems. 1 tsp ~ 5mL 1 tbs ~ 15mL

1 tsp ~ 60 gtts 1 cup = 8 fl oz 16 fl oz = 1 pint 32 ounces = 1 quart or 2 pints It is critical that you memorize conversion factors, for drug dosages and calculations associated with client care. To make it easier, we have provided you with a conversion chart for use in this course. Recall that we mentioned that conversions between Metric, Apothecary and Household are approximate, not exact. Each chart you see may have slightly different conversion factors. Note, for example, that 1 mL is approximately 15 min but that 5 mL is approximately 60 min. Also note that 1 gram is approximately gr xv and that 60 mg is approximately gr i. If you do the math, you find some discrepancies, but for the purposes of this course, either calculation may be used. Review the relationships and the values in the common conversions factor chart. You must have these memorized prior to the comprehensive final exam.

CONVERTING BETWEEN SYSTEMS OF MEASUREMENTS A conversion is a change of the same type of units within the same system or from one system to another. Conversions are ONLY done between units of volume (volume to another measure of volume); or between units of weight (weight to another measure of weight); or between units of length (length to another measure of length). We are “converting” ONLY when the measures of the same type. There are many methods to calculate the conversion among and between the 3 systems of measure. Your text illustrates the ease with which conversion within the metric system can be converted. Multiples of 10s allow us to move the decimal point in one direction or another depending on whether we are moving from larger or smaller units. Remember that if we are moving from larger to smaller units, the total number of units INCREASE. If we convert from a smaller to a larger unit, the total number of units DECREASE. Note the diagrams: (Remember you must keep in mind whether you are converting from larger or smaller units and which way and how many places you will move the decimal point. If you aren’t ready for this shortcut yet…the math will tell you what to do next.)

On the left side, units move smaller to larger

On the right side, units move larger to smaller

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